Q&A: Treated Lumber’s Moisture Content

Treated Lumber’s Moisture Content

Q.What is the moisture
content of treated lumber? The treated lumber I use
seems very wet, but it carries a grading stamp that
lists the moisture content at 19%.

A.Gene Wengert
responds: Most treated lumber carries two
stamps: The grade stamp indicates the condition
before treatment; the treatment stamp indicates the
level and type of treatment.

Before treatment, the lumber (southern pine, in
most cases) is typically dried to approximately 19%
MC (moisture content). This initial drying removes
the "free water" from the lumber, making room for
the preservatives. After pressure-treating with
chromated copper arsenate (CCA), the moisture
content will be 75% or higher.

The treatment process causes the lumber to
swell, so when you receive it at your job site, it
will be oversized. The treated lumber will shrink
when exposed to the air, just as it did when it was
first cut from the tree and dried before treatment.
Once installed, it will dry to an equilibrium
moisture content that is controlled by the
surrounding conditions. Treated lumber installed in
a relatively dry location can shrink as much 4% in
width and 2% in thickness: For a 2x12, for example,
that’s nearly 1/2 inch in width and 1/16
inch in thickness.

Some areas offer treated lumber that has been
"kiln dried after treatment" called KDAT. This
material is considerably more expensive, and you
should make sure that the final MC is
specified.

Professor Gene Wengert is
Extension Specialist in Wood Processing at the
Department of Forest Ecology and Management at the
University of Wisconsin-Madison.